The treatment of chronic and non-chronic pain conditions is very important in medicine. There is therefore a universal need for highly effective therapies for a patient-friendly and targeted treatment of chronic and non-chronic pain conditions, including the successful and satisfactory treatment of pain on the part of the patient.
Conventional opioids such as morphine are highly effective in treating severe to extremely severe pain. Their use is however limited by the known side effects such as for example respiratory depression, vomiting, sedation, constipation and development of tolerance. Also, they are less effective in treating neuropathic or incidental pain afflicting in particular tumour patients.
Opioids exert their analgesic effect by binding to cell membrane receptors that belong to the family of the so-called G protein-coupled receptors. In addition to these there are further receptors as well as ion channels that are significantly involved in the system of pain generation and pain transmission, for example the N-methyl-D-aspartate ion channel (NMDA ion channel), via which a substantial part of synaptic communication proceeds and through which the calcium ion exchange between a neuronal cell and its environment is controlled (see for example P. D. Leeson, L. L. Iversen, J. Med. Chem. 37 (1994) 4053–4067).
Important knowledge concerning the physiological importance of ion channel selective substances has been made possible by the development of the patch-clamp technique, by means of which the effect of NMDA antagonists (i.e. antagonists of the NMDA ion channel) on the calcium level in the cell interior can be detected.
In the unactivated state the NMDA ion channels are in each case closed by individual magnesium ions that are present in the interior of the channel and cannot pass through the latter on account of their size. In the activated state the smaller calcium and sodium ions can pass through the channel. The (+)-MK801 binding site of the NMDA ion channel (ionotropic NMDA receptor) is also present in the interior of this membrane protein. Substances with an NMDA antagonistic action, such as phencyclidine (PCP), ketamine or MK801, occupy this binding site (so-called “channel blockers”) and accordingly close the relevant NMDA ion channel.